• Schizotypal and borderline personality disorders (SPD and BPD, respectively) appear to be different at follow-up, yet they are poorly discriminated from each other by current DSM-III symptom criteria. In the Chestnut Lodge Follow-up Study, three axis II study cohorts (pure SPD, n =10; pure BPD, n =81; mixed SPD/BPD, n =18) with distinctive outcomes are defined using current borderline systems. This study compares the relative frequency with which individual symptom criteria from each system discriminate across study cohorts. Findings suggest that (1) for SPD, the most characteristic (core) DSM-III symptoms are odd communication, suspiciousness/paranoid ideation, and social isolation, while the least discriminating symptom is illusions/depersonalization/derealization; (2) the core DSM-III symptoms for BPD are unstable relationships, impulsivity, and self-damaging acts, while the least discriminating symptoms are inappropriate anger and intolerance of aloneness; (3) depression as a symptom does not discriminate between SPD and BPD; and (4) transient psychoses and brief paranoid experiences and/or regression in treatment discriminate for SPD but against BPD and therefore fit better as SPD criteria. Results support the retention of some, but the elimination of other, DSM-III symptom criteria for the diagnosis of SPD and BPD.